It is common to evaluate autonomic function on the basis of the results of frequency analysis (spectral analysis) of heart rate variability (variation of R-R interval) on an electrocardiogram (ECG) by using fast Fourier transform and other techniques. It is known that low frequency components (LF) up to 0.15 Hz contained in the frequency analysis results of heart rate variability predominantly reflect sympathetic function (partially including parasympathetic function), while high frequency components (HF) equal to or higher than 0.15 Hz contained in the frequency analysis results reflect parasympathetic function. The ratio (LF/HF) between the low frequency components and the high frequency components is used as an index of autonomic function.
Patent Document 1 discloses an autonomic function evaluating apparatus described below. To evaluate autonomic function and other information, this autonomic function evaluating apparatus uses an acceleration plethysmogram (APG) signal, which is obtained by taking the second derivative of a photoplethysmogram (PPG) signal, instead of an electrocardiogram signal. The autonomic function evaluating apparatus computes an acceleration plethysmogram by taking the second derivative of the waveform of a plethysmogram measured continuously over a predetermined period of time. From the obtained waveform of the acceleration plethysmogram, the autonomic function evaluating apparatus then determines variation of the a-a interval (interpulse interval) corresponding to variation of the R-R interval on an electrocardiogram. Subsequently, the autonomic function evaluating apparatus performs frequency analysis of the temporal variation of the a-a interval, and uses the results to evaluate autonomic function.
The autonomic function evaluating apparatus allows autonomic function to be evaluated by measuring the plethysmogram of the fingertip of a person in a seated position using a photoplethysmographic sensor. This eliminates the need for, for example, removal of clothing or attachment of a plurality of electrocardiographic electrodes to take a measurement, allowing the evaluation results to be obtained with greater ease compared to those obtained using an electrocardiogram.
Patent Document 1: Japanese Patent No. 5080550.
The above conventional approaches to evaluate autonomic function by heart rate variability analysis (frequency analysis) using data such as an electrocardiogram and an acceleration plethysmogram require data such as heart rate to be taken under stable conditions of the autonomic nervous system (that is, the sympathetic and parasympathetic nervous systems). This requires the user (subject) to relax in a resting seated position for about five minutes, for example, prior to taking a measurement. After taking sufficient rest, the user is then required to have his or her electrocardiogram or photoplethysmogram continuously measured in that state for the duration of, for example, three minutes or more (or, for example, one hundred beats or more).
Consequently, the conventional heart rate variability analysis may fail to correctly evaluate autonomic function in situations where the sympathetic nerve becomes dominant, such as when the subject is not allowed sufficient rest prior to measurement or when the subject is unable to remain at rest during measurement. For conventional autonomic function analysis, data such as the amount of autonomic activity (TP) or autonomic balance (LF/HF) measured after rest is known to be associated with, for example, the subjective symptoms of fatigue, the amount of activity during wakefulness, and sleep efficiency. Unfortunately, the association between autonomic function and these pieces of information is lost if a measurement is taken immediately without giving the subject a five-minute rest, seriously undermining the diagnostic value of such an autonomic function analysis. This has led to a desire for a technique that, in acquiring biological information including an electrocardiogram and a photoplethysmogram (acceleration plethysmogram), allows a biological state such as autonomic function to be estimated in a shorter time without requiring the user (subject) to remain at rest.